This invention relates to electrophoretic display devices in general and, more particularly, to an electrophoretic display apparatus which employs means for selectively erasing and rewriting individual characters on the display panel thereof.
Electrophoretic displays (EPIDS) are now well known. A variety of display types and features are taught in several patents issued in the names of the inventors herein, Frank J. DiSanto and Denis A. Krusos and assigned to the assignee herein, Copytele, Inc. of Huntington Station, N.Y. For example, U.S. Pat. Nos. 4,655,897 and 4,732,830, each entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS describe the basic operation and construction of an electrophoretic display. U.S. Pat. No. 4,742,345, entitled ELECTROPHORETIC DISPLAY PANELS AND METHODS THEREFOR, describes a display having improved alignment and contrast. Many other patents regarding such displays are also assigned to Copytele, Inc.
The display panels shown in the above-mentioned patents operate upon the same basic principle, viz., if a suspension of electrically charged pigment particles in a dielectric fluid is subjected to an applied electrostatic field, the pigment particles will migrate through the fluid in response to the electrostatic field. Given a substantially homogeneous suspension of particles having a pigment color different from that of the dielectric fluid, if the applied electrostatic field is localized it will cause a visually observable localized pigment particle migration. The localized pigment particle migration results either in a localized area of concentration or rarefaction of particles depending upon the polarity and direction of the electrostatic field and the charge on the pigment particles.
The electrophoretic display apparatus taught in the foregoing U.S. Patents are "triode-type" displays having a plurality of independent, parallel, cathode row conductor elements or "lines" deposited in the horizontal on one surface of a glass viewing screen. A layer of insulating photoresist material deposited over the cathode elements and photoetched down to the cathode elements to yield a plurality of insulator strips positioned at right angles to the cathode elements, forms the substrate for a plurality of independent, parallel column or grid conductor elements or "lines" running in the vertical direction. A glass cap member forms a fluid-tight seal with the viewing window along the cap's peripheral edge for containing the fluid suspension and also acts as a substrate for an anode plate deposited on the interior flat surface of the cap. When the cap is in place, the anode surface is in spaced parallel relation to both the cathode elements and the grid elements. Given a specific particulate suspension, the sign of the electrostatic charge which will attract and repel the pigment particles will be known. The cathode element voltage, the anode voltage, and the grid element voltage can then be ascertained such that when a particular voltage is applied to the cathode and another voltage is applied to the grid, the area proximate their intersection will assume a net charge sufficient to attract or repel pigment particles in suspension in the dielectric fluid. Since numerous cathode and grid lines are employed, there are numerous discrete intersection points which can be controlled by varying the voltage on the cathode and grid elements to cause localized visible regions of pigment concentration and rarefaction. Essentially then, the operating voltages on both cathode and grid must be able to assume at least two states corresponding to a logical one and a logical zero. Logical one for the cathode may either correspond to attraction or repulsion of pigment. Typically, the cathode and grid voltages are selected such that only when both are a logical one at a particular intersection point, will a sufficient electrostatic field be present at the intersection relative to the anode to cause the writing of a visual bit of information on the display through migration of pigment particles. The bit may be erased, e.g., upon a reversal of polarity and a logical zero-zero state occurring at the intersection coordinated with an erase voltage gradient between anode and cathode. In this manner, digitized data can be displayed on the electrophoretic display.
Accordingly, in order to erase the image in an electrophoretic display of this type, the cathode is biased positively with respect to the anode, i.e. to create an electric field of opposite polarity. In the aforementioned type of electrophoretic display device, the anode is a unitary planar structure to which one voltage is applied in the write mode and a different voltage is applied in the erase mode. All lines of the displayed image are erased simultaneously upon application of the erase voltage anode, and all lines of the display must be rewritten to form the next image frame. The next frame may often have character lines for image portions which are the same as the previous frame, which results in the redundancy of rewriting numerous identical lines from frame to frame.
U.S. Pat. No. 5,066,946, issued on Nov. 19, 1991, to Frank J. DiSanto and Denis A. Krusos, entitled "Electrophoretic Display Panel With Selective Line Erasure", which is also owned by the assignee of the present application, discloses an electrophoretic display panel which provides for selective line erasure. The electrophoretic display apparatus of this patent has grid and cathode conductors arranged as an X-Y matrix spaced from an anode with an electrophoretic dispersion in between them. Pigment particles in the dispersion become charged at selected intersection areas of the X-Y matrix and migrate towards the anode to form a display image thereon by biasing the cathode negatively with respect to the, anode, and the display image is erased by oppositely biasing the cathode and anode. The anode is formed by a plurality of parallel line segments corresponding to image lines of the display. Each anode line segment is a longitudinal rectangular conductor having a height which corresponds to the height of a text character line. Line control means are also provided and have a plurality of switch elements for individually switching the potential applied to an anode line segment to be erased from a first potential for writing to a second, different potential for erasing the line segment, while all the other line segments that are not to be erased are maintained at the first potential.
Although single line erasure is preferable to erasure of all the lines in the display, it is not an optimally efficient manner when it is desired to change only select image characters in the display.
Accordingly, it is an object of the present invention to provide a method and an electrophoretic display which overcomes the aforementioned disadvantages of the prior art devices. In particular, the object of the invention is to provide an electrophoretic display in which individual characters of the display can be selectively erased and rewritten without disturbing the other image characters which remain the same from one frame to the next.